[wip] Add canvas.toDataURL("image/png") compression control

Source/platform/image-encoders/skia/PNGImageEncoder.h

Index: Source/platform/image-encoders/skia/PNGImageEncoder.h
diff --git a/Source/platform/image-encoders/skia/PNGImageEncoder.h b/Source/platform/image-encoders/skia/PNGImageEncoder.h
index e8fc52f121c399ce101461d7c6ea8d2deea2d098..5e4732c8b304b2a090b121680454a1976241cfbf 100644
--- a/Source/platform/image-encoders/skia/PNGImageEncoder.h
+++ b/Source/platform/image-encoders/skia/PNGImageEncoder.h
@@ -39,11 +39,14 @@ namespace WebCore {
 
 struct ImageDataBuffer;
 
-// Interface for encoding PNG data. This is a wrapper around libpng.
 class PNGImageEncoder {
 public:
-    static bool encode(const SkBitmap&, Vector<unsigned char>* output);
-    static bool encode(const ImageDataBuffer&, Vector<unsigned char>* output);
+    // Encode the input data with a compression quality in [0-100].
+    static bool encode(const SkBitmap&, int quality, Vector<unsigned char>* output);
+    static bool encode(const ImageDataBuffer&, int quality, Vector<unsigned char>* output);
+
+    // For callers: provide a reasonable compression quality default.
+    enum Quality { DefaultCompressionQuality = 30 };
 };
 
 } // namespace WebCore